Merge CODEOWNERS Part 1 (paritytech#1472)

* Merge CODEOWNERS

* Add the XCM team as owner of /polkadot/xcm

* Update .github/CODEOWNERS

Co-authored-by: Alexander Samusev <41779041+alvicsam@users.noreply.github.com>

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Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Alexander Samusev <41779041+alvicsam@users.noreply.github.com>